As a semiconductor switch for a switching power source, for example, a next generation semiconductor switching device which has a low threshold voltage to be driven by a driver circuit with low DC voltage source is introduced recently. With the low threshold voltage, an erroneous operation tends to occur due to noise. For this reason, a driver circuit is required to suppress the erroneous operation. Such a driver circuit is configured typically with two switches. This driver circuit applies a positive voltage to a gate of the element by turning on one of the switches when the element is to be turned on. The driver circuit applies a negative voltage to the gate of the element by turning on the other of the switches when the element is to be turned off. According to this configuration, a gate voltage at the time of turning off largely deviates from the threshold voltage potential. As a result, even when the noise generated at the time of switching is superimposed to the gate, the erroneous operation (erroneous turning on), which will be caused by the noise, can be restricted from occurring.
The driver circuit, however, needs a power source circuit which provides a negative power source in addition to a power circuit which provides a positive power source. It is thus difficult to reduce a size of an entire circuit and reduce manufacturing cost. To address the problem, a driver circuit which is capable of generating a negative voltage in its operation, is proposed in, for example, JP 4682173.
The driver circuit of this configuration turns on one of its switches when a semiconductor switching device, which is a control object, is to be turned on. The driver circuit thus forms a current flow path which extends from a high-potential side output terminal of a drive power source to a gate of the semiconductor switching device through the turned-on switch. At this time, the gate of the semiconductor switching device is supplied with a voltage, which is equal to a voltage (power source voltage) of the drive power source. The driver circuit turns on two switches when the semiconductor switching device is to be turned off. The driver circuit thus forms a current flow path which extends from the gate of the semiconductor switching device to a low-potential side output terminal of the drive power source through the two switches and a capacitor. At this time, the capacitor is charged with electricity equivalent to the power source voltage. Hence, the gate of the semiconductor switching device is supplied with a negative voltage, absolute value of which is equal to the power source voltage. To summarize, the driver circuit described above operates with a power source and still can thus apply to the gate of the semiconductor switching device the positive and negative voltages having the absolute values equal to the power source voltage.
The driver circuit configured as described above has one switch in the current flow path (gate charge path) which extends to the gate of the semiconductor switching device when the semiconductor switching device is turned on. The driver circuit has two switches in the current flow path (gate discharge path) which extends to the gate of the semiconductor switching device when the semiconductor switching device is turned off. This driver circuit thus includes three switches for passing the gate charge electricity and the gate discharge electricity, whereas the driver circuit having the typical configuration described above has two switches for passing the gate charge electricity and the gate discharge electricity. Consequently, the driver circuit having the above-described configuration needs one more switch for charging and discharging the gate relative to the driver circuit of the typical configuration.
In driving the semiconductor switching device, it is necessary to change the gate potential of the semiconductor switching device at high speeds to realize high speed switching operations. When the gate potential is changed at high speeds, the currents for charging and discharging the gate increase relatively. Therefore, the switches provided in the gate charge path and the gate discharge path are required to allow sufficiently large currents for charging and discharging. Thus the switches are necessarily designed to have large current supply capability.
For those reasons, the number of switches provided in the gate charge path and the gate discharge path in the driver circuit is a very important factor to determine a circuit occupying area (semiconductor chip area when integrated). If the number of the switches provided in the gate charge path and the gate discharge path increases as in the driver circuit described above, the circuit occupying area need be increased and hence the manufacturing cost will increase.